1. Field of the Invention
The present invention relates to tape automated bonding processes and more particularly to the compliant pad or work piece for use in the inner lead bonding process.
2. Background Information
In the tape automated bonding process (TAB Bonding), there are two bonds typically made to complete the bonding cycle. The first bond is made to attach an integrated circuit device to a carrier tape (TAB tape), and is known as inner lead bonding. The second bond, known as an outer lead bond, is made while attaching the bonded device to a substrate assembly, and is done after the inner lead bonded device is excised from the associated carrier tape.
The inner lead bonding process requires that a heated tool (thermode) be used to clamp together the tape automated bonding process tape and the device to be bonded. A number of problems arise during this procedure, such as lack of planarity, variation in heat distribution, and of particular interest in the present application is the shielding of the device by the thermode. Previously, a number of techniques have been described for meeting with the problems of planarity and heat distribution, and providing means to effect these conditions, thus improving the inner lead bonding operation. However, the third problem, that of shielding the device by the thermode, has not been dealt with as far as is known in the technology, with the present application directed to meet that particular problem; namely, the shielding of the bond site by the thermode during the bonding cycle.
In the usual tape automated bonding process, the inner lead bonding cycle consists of placing an integrated circuit device to be bonded onto a preheated work holder, aligning a tape automated bonding tape pattern to the device, then clamping the two together and applying heat in order to create a bond.
In at least one tape automated bonding process a gold tin material system is employed wherein the gold is plated onto the device over the bonding pads to create pillars of gold known, in the industry, as gold bumps and tin is then plated to the tape. During the bonding cycle, the thermode is heated so that the interface of the gold and tin will diffuse together to form a gold/tin alloy.
Similar to the inner lead bonding procedure, the outer lead bonding operation also makes use of a reflow, or an alloying bonding technique at the interface of the beam and the substrate. Therefore, in both the inner and outer lead bonding procedures it is important to attempt to prevent surface oxidation of the tin plating. One effective way to do this during this process is to provide an inert cover gas, such as nitrogen, to flood the bonding region during the elevated bonding temperature, which is approximately 300 to 350 degrees centigrade.
The thermodes which are used for inner lead bonding must be large enough to make contact with all of the bond sites on the device. In addition, the thermode must be flat, and planar to the surface of the integrated circuit device. Experimental use has shown a need to increase the size of the thermode, so that the outer edges of the thermode extend beyond the boundary of the device to be bonded. Two reasons exist for this requirement. First, with a larger thermode it is possible to achieve a more uniform heating, and secondly, the thermode may be more evenly lapped. On small thermodes, it was found that the edges of the thermode became rounded during the lapping procedure. The rounding of the thermode's edge resulted in a reduction of the bonding area on the thermode. Therefore, to allow a larger bonding surface, the thermodes are designed so that they are larger than the required bonding area. As a result, during the bonding cycle, the oversize thermode covers and shields both the inner lead and outer lead bonding area of the tape. Under these conditions, the effectiveness of the application of the cover gas is reduced when gas is supplied from above the device.